1. Field of the Invention
This invention generally relates to a method and apparatus for the suppression of the glow-to-arc transition in DC and RF glow discharges and more specifically to a cathode configuration having a perforated dielectric covering for stabilizing glow plasma discharges.
2. Related Art
A “plasma” is a partially ionized gas composed of ions, electrons, and neutral species. This state of matter is produced by high temperatures or strong electric fields created by constant or pulsed DC current, AC current or time varying (e.g., R.F. or microwave) electromagnetic fields. Discharge plasmas are produced when free electrons are energized by electric fields in a background of neutral atoms/molecules. These electrons cause electron-atom/molecule collisions which transfer energy to the atoms/molecules and form a variety of species which may include photons, metastables, atomic excited states, free radicals, molecular fragments, monomers, electrons, and ions. The neutral gas becomes partially (or fully) ionized and is able to conduct currents. The plasma species are chemically active and/or can physically modify the surface of materials and may therefore serve as the basis of new chemical compounds and may be used to modify existing compounds. Discharge plasmas can also produce useful amounts of optical radiation and can therefore be used in lighting. There are additionally many other uses for such plasmas. Glow discharges and arc discharges produce a class of plasmas known as current-maintained plasmas, since they are maintained by the passage of current therethrough. Such plasmas conduct only because current is passed therethrough and the conductivity falls off quickly if the source of energy to the charge carriers is removed.
Transition points exist at which the various attributes of the discharge and discharge plasma change from the characteristics of a glow discharge to the characteristics of an arc discharge. The characteristics that distinguish arc from glow are a high gas temperature and a low cathode fall potential, though it is also possible to have a high gas temperature associated with a high cathode fall and vice versa.
The transition from glow to arc passes through a series of stable or quasi-stable states. However, the final step from abnormal glow to arc is very often an unstable change, since a very large potential drop in the series resistance would be required to make it stable. If there is no series resistance, the transition may take place very rapidly, without equilibrium being achieved in any intermediate stage. This transition becomes more rapid as the pressure of the background neutral gas increases towards atmospheric pressure.
In the past, there have been efforts to stabilize glow plasma discharges in various ways such as the use of source frequencies over 1 kHz, insertion of a dielectric plate (or plates) between two metal electrodes and by using helium dilution gas. Additionally, other attempts to stabilize the glow plasma discharge include placement of an insulated plate on the lower electrode, use of a brush-style upper electrode, and the use of a metal upper plate in combination with an insulating plate on the bottom thereof. However, there are certain drawbacks with these requirements in that, e.g. helium is expensive and there are physical limitations based on the structure of the electrodes and the insulated plates.
Past work in this area include a series of articles by Okazaki, Satiko, et al., starting back in 1989 with the article by Kanazaw, S. et al., entitled, “Glow Plasma Treatment at Atmospheric Pressure for Surface Modification and Film Deposition,” Nuclear Instruments and Methods in Physics Research (1989) Elsevier Science Publishers, B.V. (North-Holland Physics Publishing Division), which disclosed a plasma treatment at atmospheric pressure to stabilize glow plasma by treatment in a gas which includes carbon-tetrafluoride (CF4), using helium as the dilute gas and using an insulating plate on a lower electrode, and using a brush style electrode for the upper electrode to create a stable discharge at 3,000 Hz.
Yokoyama, T, et at., “The improvement of the atmospheric-pressure glow plasma method and the deposition of organic films,” Journal of Physics (1990) IOP Publishing, Ltd., discloses an improved atmospheric pressure glow discharge plasma method for treating metallic substrates wherein the middle plate upper electrode is improved by use of an insulating plate set on its bottom.
Yokoyama, T, et al., “The mechanism of the stabilization of glow plasma at atmospheric pressure,” Journal of Physics (1990) IOP Publishing, Ltd., discloses stabilization of a glow discharge of atmospheric pressure by controlling three conditions, namely, the use of a high frequency source, the use of helium gas for dilution, and the insertion of a dielectric plate between electrodes.
Okazaki, Satiko, et al., “Appearance of stable glow discharge in air, argon, oxygen, and nitrogen at atmospheric pressure using a 50 Hz source,” Journal of Physics, (1993) IOP Publishing, Ltd., discloses a method and apparatus for stabilizing glow discharge by making the discharge occur in the early stages of the Kekez curve, and at a lower discharge breakdown voltage, by use of a metal wire mesh electrode.
Kogoma, Masuhiro, et al., “Raising of ozone formation efficiency in a homogeneous glow discharge plasma at atmospheric pressure,” Journal of Physics (1994) IOP Publishing, Ltd. discloses an ozone formation apparatus for increasing the efficiencies of ozone formation by use of a homogenous glow discharge at atmospheric pressure to create ozone efficiencies increased to about 10% in air to a maximum of 15% in oxygen over conventional filamentary current discharges in gas. The increase is attributed to better collision efficiency among electrons and molecules and to a lower increase in temperature than in discharge filaments of a silent electric discharge.
Other work in this area includes U.S. Pat. No. 4,698,551 to Hoag, entitled, “Discharge Electrode for a Gas Discharge Device,” which uses pin-shaped electrodes which are effectively cooled in the glass flow and which promote a stable glow-discharge.
U.S. Pat. No. 5,387,842 dated Feb. 7, 1995 to Roth, et al., entitled, “Steady-State, Glow Discharge Plasma,” and U.S. Pat. No. 5,414,324 dated May 9, 1995 to Roth, et al., entitled “One Atmosphere, Uniform Glow Discharge Plasma,” both disclose a steady state glow discharge plasma generated between a pair of insulated metal plate electrodes spaced up to five centimeters apart and energized with a RMS potential of 1 to 5 KV at 1 to 100 Khz. The space between the electrodes is occupied by a noble gas such as helium, neon, argon, etc., and it may also include air. The radio frequency amplifier means for generating and maintaining a glow discharge plasma includes an impedance matching network. The arc of electric field is high enough to trap the positive ions of the plasma between the electrodes, but not so high that the electrons of the plasma are also trapped during a half cycle of the RF voltage.
None of these previous efforts disclose all of the benefits of the present invention, nor does the prior art teach or suggest all of the elements of the present invention.